In ink jet printing systems the print is made up individual droplets of ink generated at a nozzle and propelled towards a substrate. There are two principal systems: drop on demand where ink droplets for printing are generated as and when required; and continuous ink jet printing in which droplets are continuously produced and only selected ones are directed towards the substrate, the others being recirculated to an ink supply.
Continuous ink jet printers supply pressurised ink to a print head assembly, which has a heater for raising the temperature of the ink to a controlled temperature and a drop generator where a continuous stream of ink emanating from a nozzle is broken up into individual regular drops by an oscillating piezoelectric element. The drops are directed past a charge electrode where they are selectively and separately given a predetermined charge before passing through a transverse electric field provided across a pair of deflection plates. Each charged drop is deflected by the field by an amount that is dependent on its charge magnitude before impinging on the substrate whereas the uncharged drops proceed without deflection and are collected at a gutter from where they are recirculated to the ink supply for reuse. A phase measurement system is also usually present as part of deflection plate assembly and is used to ensure synchronisation of deflection for the droplets. The charged drops bypass the gutter and hit the substrate at a position determined by the charge on the drop and the position of the substrate relative to the print head assembly. Typically the substrate is moved relative to the print head assembly in one direction and the drops are deflected in a direction generally perpendicular thereto, although the deflection plates may be oriented at an inclination to the perpendicular to compensate for the speed of the substrate (the movement of the substrate relative to the print head assembly between drops arriving means that a line of drops would otherwise not quite extend perpendicularly to the direction of movement of the substrate).
In continuous ink jet printing a character is printed from a matrix comprising a regular array of potential drop positions. Each matrix comprises a plurality of columns (strokes), each being defined by a line comprising a plurality of potential drop positions (e.g. seven) determined by the charge applied to the drops. Thus each usable drop is charged according to its intended position in the stroke. If a particular drop is not to be used then the drop is not charged and it is captured at the gutter for recirculation. This cycle repeats for all strokes in a matrix and then starts again for the next character matrix.
The heater in the print head assembly ensures that the viscosity of the ink, which varies with the ink temperature, is maintained at a value such that the drop generator in the print head assembly works effectively. If the ink is too viscous, because its temperature is too low, or too thin, because it is too hot, then the ink stream will not break up into suitable droplets.
Ink is delivered under pressure to the print head assembly from an ink supply system that is generally housed within a sealed compartment of a cabinet that includes a separate compartment for control circuitry and a user interface panel. The system includes a main pump that draws the ink from a reservoir or tank via a filter and delivers it under pressure to the print head assembly. As ink is consumed the reservoir is refilled as necessary from a replaceable ink cartridge that is releasably connected to the reservoir by a supply conduit. The ink is fed from the reservoir via a flexible delivery conduit to the print head assembly. Electrical power to operate the heater in the print head assembly and the drop generator are supplied by power supply system cables, typically forming part of the supply conduit. The unused ink drops captured by the gutter are recirculated to the reservoir via a return conduit, typically located as part of the supply conduit, by a pump. The flow of ink in each of the conduits is generally controlled by solenoid valves and/or other like components.
As the ink circulates through the system, there is a tendency for it to thicken as a result of solvent evaporation, particularly in relation to the recirculated ink that has been exposed to air in its passage between the nozzle and the gutter. In order to compensate for this “make-up” solvent is added to the ink as required from a replaceable ink cartridge so as to maintain the ink viscosity within desired limits when the ink is at the correct operating temperature. This solvent may also be used for flushing components of the print head assembly, such as the nozzle and the gutter, in a cleaning cycle.
It will be appreciated that circulation of the solvent requires further fluid conduits and therefore that the ink supply system as a whole comprises a significant number of conduits connected between different components of the ink supply system and the print head assembly. Ideally, the print head assembly is as small as possible to allow for flexibility of use, and will contain both the heater and the droplet generator, charge electrode, deflector plates, phase measurement system and gutter, as well control valves for controlling the flow of ink and solvent, conduits connecting these to the ink supply system and to each other, and electrical connectors to supply power to the various components. Typically, conduits for ink and solvent, as well as connections for the control system (usually electrical connections but other control systems such as hydraulic control systems could be used), and power supply cables, are bundled together to form a supply conduit leading from the printer cabinet to the print head assembly.
The many connections between the components and the conduits within the print head assembly all represent a potential source of leakage and loss of pressure. Moreover, the complexity and compactness of the print head assembly mean that when a component in the print head assembly fails, it is generally necessary to retire the print head assembly and either send it for repair or scrap and replace it, whilst an entire replacement print head assembly may have to be used to ensure continuity of production. On-site repairs would not be feasible, as the presence of multiple conduits and components in the interior of the print head assembly makes access to certain components difficult in the event of servicing or repair.
Given that continuous ink jet printers are typically used on production lines for long uninterrupted periods, reliability of parts, rapidity of repair of parts and ease of maintenance of parts may be important issues.